1. Field of the Invention
The present invention relates to vapor recovery apparatus and, in particular, apparatus consuming liquified gas to condense vapors.
2. Description of Related Art
Since the passage of the Clean Air Act, the Congress of the United States has required all persons or organizations handling hydrocarbons or chemicals whose vapors may pollute the air to install means to recover and prevent the contamination of the air by such vapors. Such contaminant can include vapors of gasoline, methylene chloride and other organic compounds.
Such vapors are generated and displaced into the atmosphere when all types of tanks are filled with liquid hydrocarbons or liquid chemicals. Such tanks may be large storage tanks, railroad car tanks, truck tanks, underground storage tanks for gasoline stations and fuel tanks on trucks, buses and automobiles. When these various types of tanks are filled with liquid hydrocarbons or liquid chemicals, vapors escape into the atmosphere and, as is well known, such vapors become a source of smog, which under certain ambient conditions produce dangerous fog conditions and so pollute the atmosphere that they produce dangerous environmental health hazards for human beings.
Known vapor recovery systems have used closed refrigeration cycles to cool a medium that is then used to condense vapors. Condensate can be drained to a decanter to separate heavy and light constituents, such as gasoline and water. The condensing coils for such units are periodically warmed or defrosted to prevent a build up of ice and frost that may block the passage of vapors through the condensing unit. See for example, U.S. Pat. Nos. 4,027,495; 4,068,710; and 4,077,789.
Such recovery units are typically designed to handle the peak flow of vapors that may be experienced during a course of a work day. To accommodate the peak load, the recovery units must be engineered with a relatively high capacity, which is underutilized most of the time.
Furthermore, these recovery units do not easily produce temperatures low enough to condense highly volatile vapors. Certain highly volatile pollutants cannot be recovered at all unless extremely low or cryogenic temperatures are reached. Even if extremely low temperatures are not physically necessary, keeping the concentration of exhausted pollutants below a mandated maximum may dictate using extremely low temperatures to reach a high recovery rates. As an example, many states and local jurisdictions have lowered the permissible concentration of gasoline vapors in stacks from 35 mg/L to 10 mg/L.
Known vapor recovery systems have employed a condenser into which liquid nitrogen is sprayed. In this system, the liquid nitrogen is mixed with the vapors, which condense because of the low temperatures of the evaporating liquid nitrogen. The gases treated in the condenser including the gaseous nitrogen are then routed through a recuperator to be warmed by the gases flowing to the inlet line to the condenser. A disadvantage with this system is the fact that the liquid nitrogen is contaminated with the target vapors and cannot be used in subsequent chemical processes that may require pure nitrogen gas.
U.S. Pat. No. 4,982,512 shows a system for recovering solvent vapors in a multiple stage recovery system. One of the stages evaporates liquid nitrogen in a heat exchanger. The nitrogen vaporized in the heat exchanger is used as an inert atmosphere in an oven. The treated gas flow that has been cleaned of vapors is relatively cold but that temperature extreme is not recovered to enhance system efficiency.
U.S. Pat. 4,551,981 shows a three stage vapor condensing system with a final stage using a condensation column in which liquid nitrogen is evaporated. The evaporated nitrogen is then forwarded to an upstream stage for precooling. A disadvantage with this system is that the nitrogen is mixed with the solvent vapors and are therefore so contaminated as to be unusable in other processes that may require pure nitrogen.
U S. Pat. Nos. 3,857,251; 4,320,627; and 4,604,115 show systems employing liquid nitrogen to condense vapors. U.S. Pat. No. 3,802,212 shows the metering of nitrogen gas through cooling coils to refrigerate a truck.
U.S. Pat. No. 4,017,283 shows the use of nitrogen in a closed cycle for condensing vapors. In U.S. Pat. No. 3,347,055 hydrogen is liquified by including a nitrogen refrigeration cycle. In U.S. Pat. No. 3,874,185 natural gas is liquified by a system including a closed nitrogen refrige cycle. See also U.S. Pat. Nos. 4,380,907; 4,575,386; 4,620,962.
In U.S. Pat. No. 3,807,396 the inlet and outlet of a cryogenic reservoir is passed through opposite sides of a heat exchanger to effectively precool the incoming flow and moderate the temperature of the outgoing flow. This reference however is unrelated to vapor condensation and recovery.
In all these systems, the liquified gas and the cleaned gas flow is not is not handled in a way that optimizes efficiency and economizes consumption of liquified gas.